Electrochromism refers to the ability to change the optical properties of a material upon application of a potential. The tunability of the optical properties of conjugated polymers as a function of potential makes them very useful electrochromic materials. Some of the key advantages of conjugated polymers over inorganic electrochromic materials include wide range of color tunability, ease of processing, low operational voltages and extraordinary color retention.
Both organic and inorganic materials have been used to design and construct electrochromic devices which work as electrochemical cells; the most simple configuration for one of these devices consist of a four layer assembly, a transparent electronic conductive film (usually ITO) covered by the electroactive material (organic or inorganic), an ionic conductive medium, and another transparent electronic conductive film to complete the cell. The use of only one electroactive film leads to short-life devices. Shortened device life is believed to be caused by the lack of a second electroactive film working as counter electrode; a second electrode would be able to complete the redox process in the cell and reduce or eliminate degradative reactions in the electrolyte. This issue has been addressed by the use of dual-type configurations, in which a fifth ion-storage layer, which could also have electrochromic function, is included in the assembly to work as counter electrode. This configuration could be used to obtain two mixed colored states or, using two materials with complementary optical characteristics, enhance the contrast between the previously defined states.
Among all the efforts made to obtain all possible combinations of colors in electrochromic devices, the possibility of switching between a transparent and a colored state has attained a lot of attention for its direct application in systems in which a control over the intensity of the light radiation passing through the device is pursued. Materials with the above characteristics can be incorporated into intelligent windows capable of providing constant light intensity irrespective of external illumination. Based on this property conjugated polymers have been proposed and demonstrated as potential candidates for rearview mirrors that eliminate glares, smart optical attenuation systems which could be useful in architectural windows, goggles, and helmets.
The common feature of all these applications is that they are able to modify the conditions in which the visible light is perceived by human eye in a fixed situation. The human eye is a factor in the development of these devices because there are substantial differences in the way the eye perceives the visible spectrum under different conditions. Photopic and scotopic visions may be considered when making such systems.
There are two kinds of light receptors in human eye: cones and rods. The former is sensitive to even small radiations but cannot perceive the different colors and are used under low illuminations and the latter can yield perceptions of the different colors, but need more radiation to get activated and are predominantly used in well illuminated conditions. The scotopic vision refers to situations wherein the cones are used under poorly illuminated situations and photopic vision is related to the use of rods under well-lit conditions. In each situation the eye is more sensitive to some wavelengths and less to others, so each wavelength has a relative weight in the overall spectrum perceived by the eye. The relative values for each wavelength have been standardized by the Commission Internationale de l'Eclairage (CIE).
There is a continued need for electrochromic devices with high switching speeds, that can be used near a subjects eyes, and that have good contrast between colored or transmissive states. There is a continued need for gel-electrolytes for electrochromic systems that are able to transport the ions efficiently and are highly transparent.